A very good way to control stress is something called Being Mindful. This means that you stop worrying about what might happen in the future, and concentrate on what's going on right now.
When you start worrying, say to yourself "Stop." Then concentrate on the
world around you. (Do this next part in your head, because it makes
people worry if you talk out loud to yourself,) List five things that
you see. Then list five things that you hear. Then list five things that you smell. Then list five things that you can feel
around you or on your skin. (You can either do five tastes or just skip
it, since most people don't notice taste unless they are eating or
drinking.) If you are still worried, go through the list again with new
things for each step.
Being mindful makes you live in the moment without stressing out about what may or may not happen later.
Another good way to relax is to decide if your worries are realistic
or not. If they are realistic, then decide what you can do to change
things. If they are not, then let go of the thought and find something
to distract you from worrying.
For example, worrying about whether you will pass your math test is
pretty realistic. You can change things by studying or asking someone to
help you understand it better. Worrying about whether people will like
you or not may be realistic - if you don't bathe or you make fun of
everyone all the time or you are a bully, then people probably won't
like you and you can change all of that. However, if you are being the
best "you" that you know how to be then you need to understand that realistically,
people don't like everyone they meet, and that's OK. You will have some
people who do like you, and some who don't. So you can let go of that
worry and remind yourself of the people you know who do like you.
A therapist, or counsellor is a good person to talk to if your worrying
seems to be taking over your life. Anxiety can be cured by therapy!